In mobile communication, ARQ (Automatic Repeat reQuest) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to “base station”) to radio communication mobile station apparatuses (hereinafter abbreviated to “mobile stations”). That is, mobile stations feed back response signals representing error detection results of downlink data, to the base station. Mobile stations perform a CRC (Cyclic Redundancy Check) detection of downlink data, and, if CRC=OK is found (i.e. if no error is found), feed back an ACK (ACKnowledgement), or, if CRC=NG is found (i.e. if error is found), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station. These response signals are transmitted to the base station using uplink control channels such as PUCCH's (Physical Uplink Control CHannels).
Also, the base station transmits control information for notifying resource allocation results for downlink data and uplink data, to mobile stations. This control information is transmitted to the mobile stations using downlink control channels such as PDCCH's (Physical Downlink Control CHannels). Each PDCCH occupies one or a plurality of CCE's (Control Channel Elements). The base station generates PDCCH's per mobile station, allocates CCE's to be occupied by the PDCCH's according to the number of CCE's required for control information, maps the control information on the physical resources associated with the allocated CCE's, and transmits the results.
For example, in order to satisfy desired received quality, an MCS (Modulation and Coding Scheme) of a low MCS level needs to be set for a mobile station that is located near the cell boundary where channel quality is poor. Therefore, the base station transmits a PDCCH that occupies a larger number of CCE's (e.g. eight CCE's). By contrast, even if the MSC of a high MCS level is set for a mobile station that is located near the center of a cell where channel quality is good, it is possible to satisfy desired received quality. Therefore, the base station transmits a PDCCH that occupies a smaller number of CCE's (e.g. one CCE). Here, the number of CCE's occupied by one PDCCH is referred to as “CCE aggregation size.”
Also, a base station allocates a plurality of mobile stations to one subframe and therefore transmits a plurality of PDCCH's at the same time. In this case, the base station transmits control information including CRC bits scrambled by the mobile station ID numbers of the destination, so that the destination mobile station of each PDCCH can be identified. Further, the mobile stations decode CCE's to which PDCCH's may be mapped, and perform CRC detection after descrambling the CRC bits by the mobile station ID numbers of those mobile stations. Thus, mobile stations detect the PDCCH's for those mobile stations by performing blind decoding of a plurality of PDCCH's included in a received signal.
However, if a larger total number of CCE's are present, the number of times a mobile station performs blind decoding increases. Therefore, for the purpose of reducing the number of times a mobile station performs blind decoding, a method of limiting CCE's targeted for blind decoding on a per mobile station basis is studied (see Non-Patent Document 1). With this method, a plurality of mobile stations are grouped, and CCE fields being CCE's targeted for blind decoding are limited on a per group basis. By this means, the mobile station of each group needs to perform blind decoding of only the CCE field allocated to that mobile station, so that it is possible to reduce the number of times of blind decoding. Here, the CCE field targeted for blind decoding by a mobile station is referred to as “search space.”
Also, to use downlink communication resources efficiently without signaling to notify PUCCH's for transmitting response signals, from the base station to the mobile stations for transmitting response signals, studies are underway to associate CCE's and PUCCH's on a one-to-one basis (see Non-Patent Document 2). According to this association, each mobile station can decide the PUCCH to use to transmit a response signal from that mobile station, from the CCE associated with the physical resource on which control information for that mobile station is mapped. Therefore, each mobile station maps a response signal from that mobile station on a physical resource, based on the CCE associated with the physical resource on which control information for that mobile station is mapped.    Non-Patent Document 1: 3GPP RAN WG1 Meeting document, R1-073996, “Search Space definition: Reduced PDCCH blind detection for split PDCCH search space”, Motorola    Non-Patent Document 1: 3GPP RAN WG1 Meeting document, R1-073620, “Clarification of Implicit Resource Allocation of Uplink ACK/NACK Signal”, Panasonic